Aluminum is commonly produced by the electrolysis of Al.sub.2 O.sub.3 at about 900.degree. C. to 1000.degree. C. Aluminum oxide being electrolyzed is generally dissolved in molten Na.sub.3 AlF.sub.6 (cryolite) that generally contains additives helpful to the electrolytic process such as CaF.sub.2, AlF.sub.3 and LiF.
In the electrolytic cell, reduction of the aluminum oxide occurs at a cathode generally positioned upon the bottom or floor of the electrolytic cell. Oxygen is liberated from electrochemically disassociating Al.sub.2 O.sub.3, and in commercial cells, generally combines with carbonacious material comprising the cell anode and is emitted from the cell as CO and CO.sub.2.
In many commercial cells, the cathode is comprised of a material relatively resistant to corrosive effects of contents of the cell such as cryolite. This cathode often covers substantially the entire floor of the cell which typically can be 6 feet wide by 18 or more feet in length.
Molten aluminum is a substance relatively resistant to corrosive and solvating effects in an aluminum electrowinning cell. In utilizing aluminum for cathode purposes in a cell, typically the cathode is an assembly including a cathodic current feeder covered by a pool of aluminum ranging in depth, depending upon the cell, from a few inches to in excess of a foot. The aluminum pool functions effectively as a cathode and also serves to protect current feeders made from materials less than fully resistant to cell contents. For example, unprotected graphite used as a cathode can generate aluminum carbide an undesirable contaminant, while when used as a covered current feeder, no such contamination results.
These pool type cell cathode assemblies contain conductive current collectors. Where these conductive current collectors are utilized in some cell configurations, these collectors contribute to an electrical current flow within the cell that is not perpendicular to the cell bottom. These nonperpendicular electrical currents can interact with strong electromagnetic fields established around cells by current flow through busses and the like contributing to strong electromagnetic fluxes within the cell.
In cells employing a pool of aluminum covering the cathode floor of the cell, the cryolite, containing the Al.sub.2 O.sub.3 to be electrolyzed, floats atop this aluminum pool. The cell anodes are immersed in this cryolite layer.
It is important that these anodes do not contact the aluminum pool, for such contact would result in a somewhat dysfunctional short circuit within the cell. The electromagnetic flux within the cell contributes to the formation of wave motion within the aluminum pool contained in the cell, making prediction of the exact depth of the aluminum pool, and therefore the minimum necessary spacing between the anode and cathode current collector and between the anode and the interface between aluminum and cryolite at any particular cell location somewhat imprecise. Therefore, cell anodes are positioned within the cryolite to be substantially above the normal or expected level of the interface between cryolite and aluminum within the cell.
The combination of a substantial aluminum pool depth and a positioning of the anodes above the cryolite-aluminum normal interface position to forestall short circuits triggered, for example, by wave motion in the aluminum that would locally alter the aluminum pool depth, establishes a substantial gap between the anode and cathode in most conventional cells. A portion of the electrical power consumed in operation of the cell is somewhat proportional to the magnitude of this gap. Substantial reductions in the magnitude of this gap would result in considerable cost savings via reduced cell electrical power consumption during operation.
In one proposal, a packing or filler material is introduced into the cell, generally to a depth normally occupied by the aluminum pool. The packing tends to break up wave motion within the cell making prediction of the position of the interface between the aluminum pool and the cryolite more predictable. Where the interface position is more reliable, the anodes can be positioned somewhat closer to the interface, promoting incrementally reduced power consumption.
In such packed cells, however, the anode and cathode remain separated by a depth of cryolite, sufficient to forestall short circuits caused by localized disruptions in the aluminum pool depth existing notwithstanding the packing. This separation can lead to a large electrical power inefficiency in operating the aluminum electrowinning cell. Further, materials used for packing the cell must be substantially resistant to corrosive effects of cell contents. Such materials often are costly, and therefore packing the large numbers of these spacious electrolytic cells necessary for producing aluminum can be economically burdensome.
Another proposed solution has been to employ so-called drained cathodes in constructing aluminum electrolysis cells. In such cells, no pool of aluminum is maintained upon a cathode current feeder to function as a cathode; aluminum drains from the cathode as it forms to be recovered from a collection area. In drained cathode cells, without wave action problems attendant to the aluminum pool, the anode and the cathode may be quite closely arranged, realizing significant electrical power savings.
In these drained cathode cells, however, the cathode or vulnerable cathodic current feeder often is in generally continuous contact with molten cryolite. This aggressive material, in contact with a graphite or carbon cathode, contributes to material losses from the cathode as well as the formation of aluminum carbides, a dysfunctional impurity. Carbon or graphite for use as a drained cathode material of construction is therefore of quite limited utility due to service life constraints.
Other longer lived materials are, in theory, availabe for use in a drained cathode. Generally these materials are both conductive and aluminum wettable refractory materials such as TiB.sub.2. It has been found that unless TiB.sub.2 and similar materials are in essentially pure form, they too lose material or corrode at unacceptable rates in the aggressive cell environment. It is believed that the molten cryolite contributes to TiB.sub.2 corrosion by fluxing reaction products of TiB.sub.2 and aluminum generated near grain boundaries of the material. While it is known that essentially pure TiB.sub.2 does not exhibit in aluminum electrowinning cells as substantial a corrosion susceptibility as does lower purity TiB.sub.2, cost and availability factors seriously limit the use of TiB.sub.2 sufficiently pure to withstand the aggressive cell environment.